The present invention relates generally to a compensation circuit for synchronizing image motion with a movable recording medium and more specifically to a compensation circuit for use in a line scan type recording apparatus.
In a line scan recorder, a recording medium, such as photo-sensitve paper, is moved in a first direction at a constant velocity while a recording beam, such as an electron beam from a cathode-ray tube, is scanned back and forth on the recording medium in a direction perpendicular to the movement of the recording medium. The intensity of the recording beam is modulated by input data producing a gray-scale image on the recording medium.
One example of a prior art recording apparatus uses photo-sensitive paper as the recording medium and an electron beam of a fiber-optic cathode-ray tube (FOCRT) as the recording beam. In such a line scan recorder, it is important to maintain the photo-sensitive paper at a constant velocity to obtain high quality recording images. Although the photo-sensitive paper may travel at a constant average speed, it is difficult to avoid short term paper velocity changes or errors due to mechanical characteristics of the paper feed mechanism. When the photo-sensitive paper moves faster, the line density is reduced and the image becomes lighter. Likewise when the photo-sensitive paper moves slower, the line density is increased and the image becomes darker. These short term changes in the velocity of the photo-sensitive paper appear as light and dark bands on the recorded images.
One solution to the problem is disclosed in U.S. Pat. No. 4,172,259. FIG. 1 is a simplified block diagram of a conventional line scan recorder with compensation means shown in the prior art. Photo-sensitive paper 6 is drawn out of paper drum 16 by drive roller 8 under the control of paper drive means 4 and moved at a constant average velocity across the face plate 14 of FOCRT 2. X-axis circuitry 24 produces electrical signals that cause the electron beam of FOCRT 2 to scan the faceplate 14 while at the same time Z-axis circuitry 14 modulates the intensity of the electron beam in response to the input data. Thus, a gray scale latent image is formed on the paper 6 to be developed by heater means (not shown).
Velocity sensing means 26 includes a light emitting means, a photo sensor and an interrupter wheel interposed between the light emitting means and the photo sensor. The interrupter wheel is coupled to the drive roller 8, and has slotted openings along its circumference. The velocity sensing means 26 generates a pulse stream, the frequency of which is proportional to the angular velocity of the interrupter wheel. The angular velocity is, in turn, proportional to the paper velocity. Therefore, the frequency of the pulse streams represents the paper velocity.
Compensation means 28 includes a frequency-to-voltage converter 30, a long time constant differentiator 32 and an integrator 34. Frequency-to-voltage converter 30 and differentiator 32 produce a voltage signal proportional to the paper velocity errors, and integrator 34 converts the signal into a signal which represents the time integral of each such error. The output of integrator 34 is applied to Y-axis deflection circuit 20 to control the vertical movement of the line scans of the electron beam so as to maintain a constant relative velocity between scan lines of the recording beam and the recording medium.
A disadvantage of the prior art technique is the complexity of the compensation means. Moreover, it is not readily adaptive to a recording apparatus having an adjustable paper speed.